Unusual Thread Profile
Unusual Thread Profile
(OP)
I have recently experienced two different fatigue failure on a large diameter studs used on reciprocating mud pumps. When examining the failure, we found a very unusual thread profile that we first thought was a manufacturing defect, however we have now learned it was by design. The pumps were made in China, and they call this thread profile a Zig-Zag, or Hackle profile over there. It's basically a normal angle on one side of the thread, but the other side is flat. It's apparently used when loads are applied in only one direction to help prevent vibrations from loosening the threads. Both the stud and the nut have a similiar profile.
Has anyone heard of this, and if you have any information about where I can learn more about it I'd very much appreaciate it. I'm trying to understand how this profile might affect the load carrying properties of the joint (as compared with a more standard thread profile), and how to best determine how to preload it.
Has anyone heard of this, and if you have any information about where I can learn more about it I'd very much appreaciate it. I'm trying to understand how this profile might affect the load carrying properties of the joint (as compared with a more standard thread profile), and how to best determine how to preload it.





RE: Unusual Thread Profile
Mike Halloran
Pembroke Pines, FL, USA
RE: Unusual Thread Profile
http://www.hexagon.de/dose/dose1e.htm#2.6
RE: Unusual Thread Profile
RE: Unusual Thread Profile
RE: Unusual Thread Profile
RE: Unusual Thread Profile
Couple of reason why not:
"Normal" bolts are thread rolled, for cost and other reasons. The asymmetrical buttress thread does not lend itself to thread rolling with acceptable tool life or without developing unacceptable defects. "Cut" threads both cost more to make in large quantities and have a poorer finish and defect level (and fatigue life) than rolled threads.
Buttress threads (while very efficient for transmitting load in a single direction) are also really crummy in bending. While proper joint design will minimize bolt bending loads, they are a reality that must be considered. I once ran a test program on some titanium aerospace fasteners with a modified buttress thread form that the designer was convinced was going to revolutionize the fastener industry. While they performed well enough when exposed to "pure" (laboratory) axial tension loading, they failed miserably in fatigue when compared to the UNJ thread “control” lot when exposed to the slightest off-axis loading.
As noted in the references, the low radial forces generated by the buttress thread form (due a loaded thread flank that its nearly perpendicular to the thread axis) make it particularly well suited for joining tubular members subjected to “pure” tension loads in a single direction (e.g. oilwell casings, hydraulic system components, propeller hubs, gun breaches, etc.) Their relative efficiency has also found them use in plastic threads.